The invention relates to a method of controlling a switching device for switching between a normal switching state and a protection-switching state by a switching signal.
FIG. 1 shows a specific application of switching devices for two line trunk groups LTGX and LTGY of an exchange in a switching system, which are assigned to each other for the event of a protection switching of data streams DATX or DATY. The exchange has a plurality of such line trunk groups, assigned in each case in pairs, a dual central switching network SN, and a dual central processor CP.
The two line trunk groups LTGX and LTGY represented in each case have a processing unit ZTX and ZTY, respectively, in which the switching and controlling function of the line trunk group is realized. Furthermore, a line trunk group has interface modules DIU and SDC, which take care of connecting the line trunk group to the subscribers or other exchanges and to the central switching network, and which include in particular switching devices which can execute the protection switching of the data streams.
A more detailed description of the exchange represented in FIG. 1, and of the method in the event of a protection switching of the data stream of a line trunk group is to be found in European Patent Application EP-A10 291 791 corresponding to U.S. Pat. No. 4,901,347 hereby incorporated by reference. According to this European patent application, in the event of protection switching, but also during routine testing and also by manual input, the data stream, i.e. the entire processing channels with the exception of the signalling channels, are diverted via the assigned processing unit. This must be performed in a clock-controlled and phase-synchronous manner, as described in more detail in European Patent Application EP-A1 0 360 924 corresponding to U.S. Pat. No. 5,146,453 hereby incorporated by reference, and European Patent Application EP-A1 0 360 065 corresponding to U.S. Ser. No. 803,615 filed Dec. 9, 1991 hereby incorporated by reference, by the controlling of the signalling channel.
The switching device in the direction from and to the switching network is in this case accommodated on the interface module SDC independent of the respective control of the line trunk group, and is represented in FIG. 1 by a switch. The control of a switching device on the interface module SDC is effected by a switching signal, which can be emitted by the two mutually assigned processing units.
It is known to pass the switching signal as a static signal to the switching device and to execute the switching immediately.
This has the disadvantage that such a static switching signal can also occur in the event of a fault and prevents the required switching or, if switching has already taken place, wrongly effect switching back.